Machine tools comprise a rotatable member to which a chuck assembly may be mounted. The chuck assembly is operative to engage a workpiece on which various machining operations may be carried out. With reference to FIG. 1, the typical prior art chuck assembly 10 for these purposes comprises a body 12 with an axially aligned cylindrical cavity (not shown) having a piston slidably mounted therein. Pneumatic pressure is selectively applied to one side or the other of the piston to generate relative axial movement between the piston and the body 12. The prior art chuck assembly 10 further includes three radially aligned channels 14 in which master jaws 16 are slidably disposed. The channels 14 may be disposed in either the body 12 or a member connected to the piston. A radially outer or radially inner surface of each master jaw typically is angularly aligned to the rotational axis of the chuck assembly 10 to define a wedge surface. The prior art chuck assembly 10 further includes an actuator (not shown) that may be rigidly mounted to the body or the piston. The actuator includes inclined wedge surfaces that are slidably engaged with the corresponding wedge surfaces on the respective master jaws 16. With this construction, relative axial movement between the piston and the body will cause a corresponding axial movement between the actuator and the master jaws 16. However, the cooperating wedge surfaces of the master jaws 16 and the actuator will convert this axial movement into a radially directed slidable movement of the master jaws 16 in the respective channels 14. Gripping jaws are rigidly but removably mountable to the master jaws 16 and undergo radial movement with the master jaws 16. In the typical prior art chuck assembly 10, the actuator will move in one axial direction to urge the jaws 16 inwardly for securely gripping the workpiece or in an opposed axial direction to move the jaws 16 outwardly to release the workpiece.
The prior art chuck assembly 10 must also be rotatable to enable the machine tool to perform an appropriate machining operation on the workpiece gripped by the chuck. The rotation of the chuck, however, must not interfere with the pneumatic pressure that is selectively applied to one side or the other of the piston for gripping the workpiece. To achieve these necessary rotation and gripping functions, the prior art chuck body 12 typically is mounted to a rotatable portion of the machine tool having a bore extending axially therethrough. A rotatable air tube assembly 18 passes through the axial bore of the machine tool and is rigidly connected to the chuck body 12 for rotation therewith. The air tube assembly 18 is comprised of a plurality of generally concentric tubes for selectively delivering compressed air to one side or the other of the piston in the chuck body 12. In particular, a central portion of the air tube assembly may selectively deliver compressed air to a forward side of the piston, while the annular space around the center tube will be employed to deliver compressed air to the rearward side of the piston. The air tube assembly 18 typically will extend between 18.0 inches and 48.0 inches depending upon the geometry of the machine to which the chuck assembly 10 is mounted. The prior art air tube assembly 18 extends from the rear side of the bore of the machine tool and is mounted to an air rotary journal 20 for selectively delivering the compressed air to one side or the other of the piston. The air journal 20 will remain stationary while the chuck body 12 and the air tube assembly 18 rotate under the action of the machine tool to which they are mounted.
The typical prior art chuck 10 shown in FIG. 1 may perform well at rotational speeds of up to approximately 4,000 rpm. At higher rotational speeds, the centrifugal forces exerted on the jaws often will overcome the pneumatic forces exerted on the piston and will force the jaws outwardly to thereby loosen the grip on the workpiece. Many prior art chucks have attempted to employ complex assemblies of counterweights which are intended to offset the effects of centrifugal force in an effort to retain the inward gripping power of the jaws at higher rotational speeds. However, chucks of this type are very complex and expensive and do not always work well.
U.S. Pat. No. 4,676,516 issued to the inventor herein on June 30, 1987 and shows a chuck that is of fairly simple construction but performs well at relatively high rotational speeds. In particular, the chuck shown in U.S. Pat. No. 4,676,516 includes an actuator ring disposed concentrically about the chuck body and the jaws. The actuator ring helps to hold the jaws at radially fixed positions in response to centrifugal forces exerted thereon. Additionally, certain embodiments of the chuck shown in U.S. Pat. No. 4,676,516 include a very simple counterweight for urging the piston or the chuck body in a direction for gripping the workpiece. As rotational speeds increase, the forces exerted by the counterweight act on the chuck to at least partly offset the effects of centrifugal force. The disclosure of U.S. Pat. No. 4,676,516 is incorporated herein by reference.
Several machining operations now require even higher rotational speeds. For example, metallic discs having axially aligned cavities on one side are employed in video cassette recorders to read the magnetically encoded signals on the video cassette tape. The discs must be manufactured with extreme precision both as to smoothness and concentricity. The need for precision and the competing need for manufacturing efficiency requires machine tools that operate at very high rotational speeds. Machine tool rotational speeds of 8,000 rpm often are required, and speeds in the range of 10,000 rpm - 12,000 rpm are considered to be more desirable. A control factor in these prior art machining operations has been the ability of prior art chuck assemblies to accurately retain the workpieces at these high rotational speeds. Although the chuck shown in U.S. Pat. No. 4,676,516 has proved to be effective at rotational speeds approaching those specified above, it has now been found that significant machining inaccuracies are attributable to the rotatable air tube assembly. More particularly, small dimensional inaccuracies are inevitable in the long thin-walled tubes. Even a carefully manufactured tube is likely to acquire a nonconcentric configuration somewhere along its length either during storage, installation or use. Even small eccentricities in any one of the purportedly concentric tubes will cause significant vibrations in the prior art chuck assembly as the high rotational speeds are approached. The probability of an eccentricity and the vibration causing effect of the eccentricity increases as the length of the air tube assembly increases. These vibrations more than offset the precision and efficiencies that can be obtained by machining at higher rotational speeds.
A chuck assembly that is very effective at lower rotational speeds is shown in U.S. Pat. No. 4,349,207 which issued to the inventor herein on Sep. 14, 1982. The chuck shown in U.S. Pat. No. 4,349,207 avoids air tube assemblies entirely and selectively applies the pneumatic fluid to one side or the other of the piston from a location adjacent the front working end of the chuck, and forward of the machine tool to which the chuck is mounted. The disposition of air supply means at a radially outward position in close proximity to the rotating gripping components of the chuck create eccentricities and unequal forces that prevent the chuck of U.S. Pat. No. 4,349,207 from operating at very high rotational speeds. These limitations do not present problems at lower rotational speeds for which the chuck of U.S. Pat. No. 4,349,207 is extremely effective.
The prior art also includes collet type chucks where a generally frustum-shaped gripping head is defined by an alternating array of metallic gripping jaws and elastomeric members arranged around the circumference of the frustum-shaped clamping head. Gripping occurs by urging the jaws inwardly from their unbiased position. The provision of the elastomeric members facilitates the release of the collet for removing a workpiece therefrom. The collet shown in U.S. Pat. No. 4,214,766 is not intended for the high rotational speeds referred to above, and the provision of the elastomeric material therein does not overcome the effects of centrifugal force. Furthermore, the collet shown in U.S. Pat. No. 4,214,766 does not employ the prior art air tube assembly as depicted in FIG. 1 above.
In view of the above, it is an object of the subject invention to provide a chuck apparatus capable of maintaining consistent gripping forces at very high rotational speeds.
It is another object of the subject invention to provide a chuck apparatus that avoids excessive vibrations at high rotational speeds.
An additional object of the subject invention is to provide a high speed chuck apparatus that is substantially immune to eccentricities in elongated air tube assemblies.
Still another object of the subject invention is to provide a chuck that enhances the uniform gripping forces on certain workpieces at higher rotational speeds.